Commercial nuclear reactors used for generating electric power include a core composed of a multitude of fuel assemblies which generate heat used for electric power generation purposes. Each fuel assembly includes an array of fuel rods held in spaced relationship with each other by spacer grids. The fuel rods may be approximately 0.5 inches in diameter and about 12 feet long and typically comprise a hollow zirconium alloy tube, or cladding, which is filled with a stacked column of cylindrical uranium dioxide fuel pellets and provided with zirconium alloy end caps.
After multiple cycles of operation, highly localized concentrations of zirconium hydride have been observed in the cladding at locations corresponding to pellet-to-pellet gaps in the uranium dioxide fuel column. The localized concentrations are believed to be due to hydrogen migration. Hydrogen apparently peripherally migrates down the thermal gradients that arise in the cladding from "cool spots" associated with the formation of pellet-to-pellet gaps. The source of the hydrogen is believed to be the water-side corrosion process which liberates hydrogen which is, in turn, absorbed by the cladding. When the concentration of hydrogen in a local region of the cladding exceeds the solubility limit at a given temperature, a phase transformation occurs, resulting in the formation of delta- phase zirconium hydride. Such local concentrations of zirconium hydride at fuel column gaps have been confirmed by Combustion Engineering, Inc., the assignee of the present invention, in fuel rods irradiated for multiple cycles.
A high local concentration of zirconium hydride can diminish cladding performance capability. For example, at the ANS Topical meeting on Fuel Performance in April, 1991, results from a failure diagnostic program on 2-cycle fuel rods were presented which suggested a link between local zirconium hydride concentrations at fuel pellet gaps and fuel failure.
The cause of approximately 21% of the fuel rod failures experienced by Combustion Engineering, Inc. is not known. Local zirconium hydride concentrations at fuel pellet gaps are suspected as being responsible for a portion of these failures, particularly in those cases where failure occurred during the second or third cycle of operation. Therefore, any method of lessening or eliminating these fuel pellet gaps would be beneficial.
One way of decreasing the number and extent of the fuel pellet gaps that are formed during operation is to maintain a force on the fuel pellet stack. Presently, fuel pellet holddown springs are made of stainless steel. Unfortunately, after a cycle of operation, stainless steel springs lose their holddown capability at high temperatures and their effectiveness in reducing fuel pellet gaps is therefore compromised.